1. Field of the Invention
The present invention relates to a drying apparatus for drying a web of paper. More particularly, the present invention relates to a plurality of single tier, top felted dryers followed by one double tiered drying section.
2. Information Disclosure Statement
In the manufacture of lightweight paper grades, such as newsprint and fine paper, the web is dried on a series of steam-heated drying cylinders. The wet web is pressed directly onto the cylinders by a series of tensioned, permeable fabrics or felts.
In a conventional double-felted, two tier dry section, the wet web passes from one cylinder to the next in a generally serpentine fashion through long, unsupported "open draws". The majority of the water vapor that leaves the sheet or web is released in these open draws.
Problems have been experienced during operation of conventional dryer sections with regard to sheet flutter during movement of the web through such open draws.
Also, in conventional dryer sections, problems are caused by cross-directional sheet shrinkage and inefficient ventilation of evaporated water.
Additionally, conventional drying sections of the aforementioned type typically require threading ropes in order to thread a tail of the web through such dryer sections.
Some of the problems with sheet flutter, sheet shrinkage, and vapor ventilation have been solved by extending the length of the single tier, top felted, sections known in the art as "BelRun" sections, to include more of the drying cylinders. However, the primary concern with extending the number of top felted sections so far down the length of the dryer section is that an imbalance in drying on each side of the web was expected to cause a problem with sheet curl.
Typically, with the aforementioned arrangement, approximately 41 percent of the dryers are disposed in a single tier configuration, and the remaining 59 percent are arranged as two tier, double felted dryers.
The problems associated with the conventional two tier dryer sections and the extended "BelRun" dryer sections have been solved with the use of the so-called "Bel-Champ" dryer section. Bel-Champ is a common law trademark of Beloit Corporation. The Bel-Champ.TM. dryer section utilizes a series of single tier dryer sections, generally disposed horizontally, such drying sections drying alternate sides of the web.
More specifically, the web is conveyed between cylinders in the Bel-Champ.TM. dryer section using the direct support by the dryer fabrics. Two-sided drying is achieved by alternating between top felted and bottom felted sections. The web is transferred between these opposite felted sections using a unique transfer arrangement. Sheet restraint is provided using the combination of felt tension pressure against the web on the cylinders, and vacuum pressure against the web on the intermediate vacuum transfer rolls. Such continuous support of the web also permits threading of the sections without the use of threading ropes.
Nevertheless, a consideration of the aforementioned Bel-Champ.TM. geometry is that there is an added machine length resulting from having all of the dryers in a horizontal arrangement rather than in a two tier arrangement.
A further consideration of the Bel-Champ.TM. configuration is that each of the section-to-section transfers normally requires two vacuum rolls.
Additionally, another consideration of the aforementioned arrangement is that, in the event of the web breaking and wrapping a bottom felted dryer, the wrapped paper cannot be easily dumped into the basement. Rather, such broke must be manually removed from the bottom felted dryer section.
Also, in the Bel-Champ.TM. arrangement, control of curl of the web cannot be provided within a single section, but rather must be effected during movement of the web through at least two adjacent sections.
The present invention provides a unique arrangement of top felted, single tier dryer sections which end with a single, two tier section. The aforementioned arrangement provides good dryer access, efficient broke removal, direct access for operation, ropeless threading and efficient curl control.
More specifically, the dryer section of the present invention consists of a series of single-tier dryer sections with the dryers preferably disposed horizontally. The dryer sections are arranged with all the section-to-section transfers located for direct access from the main operating floor. Such transfers include the press-to-dryer section transfer, the dryer-to-dryer section transfers and the dryer-to-calender section transfer.
Broke handling and removal from all of the top felted sections is done in a downward direction, thus eliminating the need for extensive scaffolding, operator platforms and conveyors which would be required for efficient access around bottom felted sections.
Additionally, the arrangement of the present invention reduces the overall length of the paper machine due to the stacking of the dryers in the two tier section. Such arrangement also achieves high average felt wrap angles of the dryer cylinders for improved drying rates, improved drivability and improved sheet restraint.
Furthermore, the proposed dryer section provides enhanced two-sided drying for improved curl control, using the last dryers in the two-tier dryer section for such control. The last dryers have been found to be the most effective in terms of curl control.
Applicants have discovered that the single tier dryer sections, according to the present invention, are effective in reducing cross-directional shrinkage in the wet end of the dryer section. However, it has been further discovered that such single-tier dryer sections have less effect in the last dry end section.
Comparative test results from trials indicate that for a Bel-Champ.TM. type dryer section, the cross-directional shrinkage is nearly zero (0) until the web dryness reaches a level of about 65 to 80 percent dry. After this point in a single tier section, the cross-machine direction (CD) shrinkage increases, although at a rate that is less than the shrinkage rate of a web dried with a conventional dryer section, that is a double felted, two tier dryer section. Such shrinkage occurs even though the Bel-Champ.TM. dryer section is utilized. For this reason, little loss in web width will occur even though the last dryer section, according to the present invention, is arranged in a two tier configuration.
Furthermore, the two tier section provides an open draw location for the provision of a tail cutting mechanism. At the same time, the dryers in the two tier section are arranged with the felt roll offset so as to reduce the length of the open draws in order to maintain sheet stability and to direct the tail into the next felt/dryer nip in order to thread the tail without the need for threading ropes.